Gating circuit



States Patent 2,954,467 1 Patented Sept- 1960 Aircraft Company, Culver City, Calif a corporation of Delaware Filed Jan. 30, 1958, Ser. No. 712,268

3 Claims. (Cl. 250 -27) This invention relates to gating circuits and more particularly to a gating circuit especially adapted to pass low-level intermediate fiequency signals with no adverse results due to ringing.

In conventional gating circuits wherein a receiver is activated by applying a voltage excursion to an element of a tube normally cut off, thereby allowing the tube to conduct for the duration of the excursion, there results a small amount of ringing, i.e., damped oscillations, in the parallel resonant circuit which serves as a load impedance for the stage being gated. When this gating is eifected at high levels of' signals, for example, at a stage which is near the output circuit of an LP. amplifier, the amplitude of the ringing is sufficiently small compared with the amplitude of the signal being gated that is of negligible consequence and there exists no desirable results.

When the aforementioned type of gating is employed on a stage near the input stage of the LF. amplifier where the signal to be amplified is at an extremely low-level, for example, on the stage immediately subsequent to the low noise input stage of the amplifier, the amplitude of the ringin'g'associated with the gating is generally greater than the amplitude of the signal being amplified. Hence,'the ringing associated with the gating completely masks the desired signal whereby it is apparent that this type of gating is rendered useless for low level signals. Further, it has been noted that applying the gating pulse through a series or shunt diode doesnot completely eliminate the ringing whereby it is necessarily amplified in subsequent stages along with the desired signal. 7 Q It is therefore an object of the present invention to provide an improved low-levelintermediate frequency gating circuit.

Another object of the invention is to provide an apparatus capable of gating a low-level intermediate-frequency signal without the introduction of a ringing signal. In accordance with the present invention, a stage for gating purposes will be inserted immediately subsequent to the initial low noise level stage which is generally employed in pre-amplifiers or LP. amplifiers. Both the low noise level stage and the gated stage preferably include pentode type amplifier tubes although this is not absolutely essential to the successful operation of the device. Under normal or ungated operating conditions, the gated stage is provided with negative feedback of a magnitude to' reduce the transfer voltage ratio to zero. Gating is then accomplished by rendering the tube of the gated stage non-conductive whereby the signal to be gated is coupled through the feedback network to the output of the gating circuit which output may be connected to the next succeeding stage of the pre-amplifier or LP. amplifier. The above-mentioned and other features and objects of this inventionand the manner of attaining them will become more apparent by reference to the following description taken in conjunction .with the accompanying drawings, wherein:

Fig. l is a schematic circuit diagram of the device of the present invention; and

Fig. 2 is an equivalent circuit diagram of the device of Fig. 1. I

Referring now to Fig. l of the drawings, there is illustrat'ed a schematic circuit diagram of thedevice of the present invention. The device comprises tubes T and T which are preferably of the pentode type, although other types of tubes may be employed. As a pentode tube, T

includes a cathode 10, a control grid 11, a screen grid 12,

suppressor grid 13 and a plate 14. T on the other hand, includes a cathode 15, a control grid 16, a screen grid 17, a suppressor grid 18 and a plate 19. The first low noise level stage incorporating T is then provided by connecting a plate 14 of tube T through a load resistor 20 and an isolating network 21 to a source ofB+ potential. The isolating network 21 may, for example, constitute an RF choke or inductor 22 connected in series from the source of B+ potential to the resistor 20 together with capacitors 23 and 24 shunted to ground at each extremity of the inductor 22. An isolating network such as network 21 is conventionally used on low noise level stages to prevent regenerative feedback from the output stages of the amplifier. Proper bias for this stage is provided by connecting the cathode 10 of the tube T through a 2 cathode resistor 26 to a source of substantially fixed potential such as, for example, ground. The cathode resistor 26 is, in turn, by-passed by a capacitor 27 so as to maintain the cathode 10 at ground potential for radiofrequency purposes; Also, as is conventional, the

1 suppressor grid 13 is maintained at the same potential as the cathode 10 by means of a connection thereto and the screen grid 12 is connected to the junction between the load resistor 20 and isolating network 21 thereby to maintain this grid at the available B+ potential. Last,

an input to the low noise level stage is provided by connections from the input terminals to the control grid 11 of the tube T and to ground, respectively. Also, an output signal from the stage is provided by connections through a coupling capacitor 28 to the junction between load resistor 20 and plate 14 of the tube T The output signal of the low noise levelstage is applied across a coil 30 connected in series with a resistorcapacitor network 31 to ground. The signal thus developed across the coil 30 and the resistor-capacitor network 31 is applied to the control grid 16 of the tube T which tube together with its associated circuitry constitutes the gating stage of the device of the present invention. The bias for the tube T is provided by connecting the cathode 15 thereof through a variable resistor 32 to ground. In order to maintain the cathode 15 at radio frequency ground, the variable resistor 32 is by-passed by a capacitor 33. Also, as is conventional, the suppressor grid 18 is maintained at the same potential as that of the cathode 15 by means of an appropriate connection thereto and the screen grid 17 is connected to the source of B-I- potential. Lastly, the plate 19 of tube T is connected through a load resistor 34 to the source of B+ potential. An output signal from the gating stage is provided by connecting the junction between plate 19 of the tube T and the load resistor 34 through 'a coupling capacitor 35 to output terminals 36. These output terminals are in turn shunted by a coil 37 which is adapted to tune out stray capacitance that inherently exists in such a circuit. 7

In accordance with the present invention, negative feedback of such a magnitude as to make the transfer voltage ratio equal to zero is applied from the output to the input of the gating stage at times when the tube T is normally conducting. This negative feedback may be provided, for example, by connecting a feedback resister 40 from the output terminals 36 to the control grid more detail, the conductance of the resistor 40 should be substantially equal to the mutual-conductance of the tube T when pentodes are employed, to efiect a transfer volta rati 91 1 1 ero- T e m t a wesl s an s o t tube T maybe adjusted, within limits, by means of the Yatiah r is r 3.2 wh ch s i its sa hqs Q i fi- Thus, as explained above, the output signal available a th Output e ina s 31 9 the ea n s a i norm of zero amplitude when the tube T is conducting, that is, when the tube is, in effect, functioning as an amplifier. In order to gate, it is necessary to change the relative value of the mutual-conductance of the tube T with respect to the conductanc'eof resistor 40. An expedient way of accomplishing this is to render the tube T nonconductive. This may be accomplished by pulsing an appropriate element on the tube T with a voltage excursion of proper amplitude and polarity. In that the plate IQ and screen grid 17 require voltage pulses of magnitudes comparable to the B+poteutial, it is preferable to apply a pulse between the control grid,16 and the cathode 15. This may conveniently be accomplished by coupling a gating pulse source 42. to the junction between the coil 30 and the resistor-capacitor network 31. When applied in this manner, the voltage excursion of the pulse should, of course, bein a negative direction. Also, the capacitor of the resistor-capacitor network 31 should be of a size to allow-the signal developed across the coil 30 to by-pass to ground and yet be .sufiiciently small as to enable the pulse to be developed thereacross. The'resistor of the network 31, on the other hand, completes a direct-current return from the control grid 16, to ground. I 'l i In order to explain more clearly the present invention, reference is made to Fig. 2 which illustrates equivalent circuit of the alternating current portion of the apparatus shown in Fig. 1. More particularly, G K and P represent the control grid '11, the cathode l0 and the plate 14, respectively, of the pentode tube T and G K and P represent .the control grid lo, the cathode 15 and the plate 19, respectively, of the pentode tube, T In that pentode tubes have been employed, a .nodal analysis will be presented wherein the tubes T and T2 will be considered as current sources as distinguished from voltage generators. In the equivalent circuit, the tube Ti may be repnesented as having an equivalent load impedance 59 which constitutes the load resistor 2 0, the coil 3 0and a capacitor 52, representative of the stray capacitance, all connected in parallel. 'Also, the tube T may be represented as having an equivalent load impedance 54 which constitutes the load resistor 34%, the coil 37 and a capacitor a, which represents stray capacitance, all connected in parallel. Also, in an analysis of this type, the plate resistance of the tubes T and T is very large and would appear in shunt with the equivalent load impedances 50, 5 respectively, whereby it is evident that the plate resistances may be neglected. Similarly, the plate to control grid capacitance ofthe tubes T and T is very small and therefore may be neglected in the analysis; It is to be noted that the impedances introduced'by coupling capacitors 28, 35 and the by-pass capacitors 27, 33 have also been neglected. This is a permissible assumptionin that the device is primarily adapted to gate intermediate-frequency signals. Thus, the feedback resistor 40. may be represented as being connecteddirectly from the plate 19 to the control grid 16 of the tube T that is, directly from P to G in the equivalent circuit of Fig. 2. 'Furither, as is generally the case in LR amplifiers, the load irnpedances 50, 54 are, as previously specified, at par allel resonance. That is, the coils 30, 37 of impedances 50, 54 tune out the stray capacitance represented ca; pacitors 52, 56, respectively. It is thus apparent that the admittances of the load impedances $0, 54will con,- stitute pure conductance s. i

A The parameters on the equivalent circuit diagram of Fig. 2 and in the analysis may then be designated as follows:

Mutual-conductance of tube T g Mutual-conductance of tube T g Admittance of load impedanceSj) at parallel at parallel resonance g Admittance of load impedance 5}} at parallel reso- T. 7 3 C nd ietau ib feed ack r sum 1-: m Signal input Q Q. e Output signal from tube T V Output signal at output terminals36 e In a nodal analysis, the sum of cur-rents flowing into any junction must always equal the current flowing away from the junction. Thus, on the basis of the polarity indicated in the equivalent circuit oi Fig, 2, the r nts flow from nd QWa d the il 9fi ieGz areas-t l s gm1 1= 11+( 2-lg12 m l ly t e cu en flowing t a and towards the junction P5 are as follows:

5m2 1.= 2 2 -l-(' 2 +V1) 812 becomes zero when g =g In the event that T is a type dAKS which has a mutual-conductance be 4000* micro-rnho s, the feedback resistor 40 would have a resistance of 250 ohms.

' When the gating stage including tubeT is'rendered non-conductive by application of a pulse or other voltage excursion from. gating pulse source 42, g becomes equal to zero'ther'eby to result in a voltage gain. In the circuit of Fig. 1, the output signalavailable frorn the-low noise level stage including tube T is eflectively connected from the plate 14' of Tube T through the coupling capacitor 28 and the feedback resistor 40f-to the output terminals 36 during intervals when'the tube-T is rendered non-conductive by a gating pulse.

By adjusting the variable resistor .32 in the cathode circuit of the tube T for' rr iinimun i output signalwhen the tube is conducting, an err-on ratiofo f at least; 40, decibels is easily achieved without neutralining the grid-to-plate capacitance. Further, when a sine wave alternation such as pulse 69, in Fig. l, is employed as gating pulses it has been found "that the device of the present invention may be followed by an amplifier of approximately decibels gain without spikes due to ringing in the output.

Another advantage of this circuit over those in which a tube is pulsed into conduction during the time thereceiver is turned on :by the application of a gating pulse to a high gain element of the tubeis the following. In the latter type circuit, any ripples or o om a const va u a hewp at the be amplified in the IF. amplifier it these, tain frequencies which lie within the pass amplifier thereby masking the desired, signal.

, [1a the circuit of the present'inv'entiorl, on the other hand, bottom of the gating pulse can have any shape, s ncethe tube is cut on during the gating intervals and hence will not pass any of the characteristics of the gating pulse.

What is claimed is:

1. A gating apparatus comprising a source of signals to be gated; an electron discharge device including a control grid, a cathode and a plate, said control grid being coupled directly to said source of signals; means for maintaining a bias potential between said control grid and said cathode of said electron discharge device; a source of B+ potential; a 'load impedance connected between said plate and said source of B+ potential whereby said electron discharge device conducts a flow of current; a feedback resistor coupled from said plate to said control grid of said electron discharge device, said feedback resistor having a conductance substantially equal to the mutual-conductance of said electron discharge device whereby the amplitude of said signals to be gated which develop across said load impedance is substantially zero; and means coupled through a coil to said control grid of said electron discharge device for applying a voltage excursion between said control grid and said cathode of a magnitude and polarity to render said electron discharge device non-conductive for predetermined intervals of time thereby to make said signal-s to be gated available at the junction between said load intpedance and said plate during said predetermined intervals of time.

2. The gating apparatus as defined in claim 1 wherein said voltage excursion applied between said control grid and said cathode of said electron discharge device has the wave form of one alternation of a sine wave thereby to minimize ringing signals across said load im-- tpedance.

3. The gating apparatus as defined in claim 1 wherein said electron discharge device constitutes a pentode tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,289,338 Bert-ram et al. July 14, 1942 2,379,042 Shaw June 26, 1945 2,531,201 De Lange Nov. 21, 1950 2,544,340 Maxwell Mar. 6, 1951 2,831,971 Wischmcyer Apr. 22, 1958 2,868,971 Wischmeyer Jan. 13, 1959 FOREIGN PATENTS 567,011 Great Britain Jan. 24, 1945 

